1. Field of the Invention
The present invention relates generally to the field of geophysical surveying. More particularly, it concerns seismic methods and geophysical survey systems for petroleum and gas exploration that rely on an explosive seismic energy source that comprises an oxidizable metal material.
2. Description of Related Art
Seismic geophysical surveys are used in petroleum and gas exploration to map the following: stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, and basement topography. Such maps are deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between layers within the subterranean formation.
A seismic energy source is used to generate seismic waves that travel through the earth and are then reflected by various subterranean formations to the earth""s surface. As the seismic waves reach the surface, they are detected by an array of seismic detection devices, known as geophones, which transduce waves that are detected into representative electrical signals. The electrical signals generated by such an array are collected and analyzed to permit deduction of the nature of the subterranean formations at a given site.
Seismic energy sources that have been used in geophysical survey methods for petroleum and gas exploration include impact sources, gun sources, vibratory sources and explosives. The nature of output seismic energy depends on the type of seismic energy source that was used to generate it.
Fundamentally, an impact source is a weight striking the surface of the earth directly or impacting a plate placed on the earth""s surface, yielding seismic energy. A weight-drop is an example of the former type of impact source. While impact sources tend to be relatively inexpensive and simple to operate and maintain, their principal disadvantage is that they are inefficient at producing seismic energy useful for geophysical survey of deeper layers. Impact sources yield a relatively high proportion of low frequency, surface waves and output less seismic energy than other seismic energy sources.
Gun sources, like impact sources, transfer kinetic energy into seismic energy. They rely on the sudden, powerful release of a charge of pressurized gas, usually compressed air from an air gun, to generate seismic waves. Gun sources have an advantage over impact sources in that they produce more seismic energy than is possible with simple impact sources. The seismic energy generated by gun sources also tends to be of higher-frequency than that imparted by impact sources, and this helps to minimize surface wave generation and improve resolution. However, gun source equipment tends to be more bulky and expensive than simple impact sources.
Vibratory sources are also used as seismic energy sources in geophysical survey methods. Two categories of vibratory sources include those that generate seismic waves originating at the surface and those that generate seismic waves that emanate from downhole. One mechanical-hydraulic vibratory source, the Vibroseis truck, is specially designed to place all of its weight onto a large platform which vibrates. This vibration, in turn, produces seismic waves in the subterranean formation. Vibroseis trucks have been used extensively in geophysical survey methods, not just for the petroleum and gas exploration, but also for studying the evolution and development of specific geological structures (e.g. the Rocky Mountains) and fault lines. Vibratory sources tend to produce highly repeatable seismic energy. The nature of the energy delivered into the ground by vibratory sources, its amount, duration, and time of delivery, can be tightly controlled and therefore the seismic energy generated tends to be very reproducible, which is a benefit. However vibratory sources are often not suited to certain types of terrain. For example if the ground is very soft, it can be difficult to use Vibroseis trucks as a seismic energy source.
Another type of seismic energy source used in geophysical survey relies on explosives. Explosive seismic energy sources used in petroleum and gas exploration on land rely on the explosion of material placed within a subterranean formation to generate seismic waves. Typically, a hole is drilled in the ground, the explosive is placed in the hole, and backfill is piled on top of the explosive, prior to initiating the explosion. Compared on a pound for pound basis to gun sources and impact sources, explosive sources impart the highest amount of seismic energy into the ground. Explosive seismic energy sources currently being used in geophysical survey methods generally produce waves of very high frequency. They are often used when the ground conditions are such as to prevent the effective use of impact or gun sources (i.e. when the ground is extremely soft).
Many explosives used in seismic energy sources generate high gas volumes. This is a useful property in mining for moving rock, but is undesirable in seismic exploration, because it decreases the amount of usable seismic energy that is generated. Explosives that produce high volumes of gas cause much of the energy of the explosion to be lost as expanding gases force backfilled material up the borehole into which the explosive was placed. Thus, less of the energy generated by the explosion is transferred into the subterranean formation than would be theoretically possible if less energy was lost to expansion of generated gases. In addition; the sudden expansion of a large volume of gas can cause permanent deformation of the subterranean formation itself.
At present, the demand for seismic exploration methods that generate sharper energy pulses, which can result in higher resolution images, has led to sacrificing the generation of low frequency seismic waves. This loss of low frequency waves compromises the ability to image deeper targets (e.g.,  greater than 3 seconds). While Vibroseis has been used successfully in mapping deeper targets, it has been difficult to achieve the same quality of results using explosive seismic sources. This presents a significant problem when there is a need for mapping deeper subterranean formations but the ground conditions are not suited to Vibroseis. In the past, the response has been to drill deeper boreholes and use more explosive to achieve the desired results at such difficult mapping sites. Both drilling deeper and using more explosive substantially increase the cost of subterranean mapping of a particular site.
There is a need for improved seismic methods and geophysical survey systems that rely on explosive compositions that convert a higher percentage of the potential energy in the explosive composition into seismic energy . There is also a need for improved methods and systems that efficiently generate low frequency seismic waves when needed. Furthermore, it would be advantageous to be able to use shallower boreholes and less explosive to achieve the necessary level of data resolution for geophysical survey.
This invention provides improved seismic methods and geophysical survey systems that are well suited for petroleum and gas exploration, but could be used for other purposes as well.
One aspect of the invention is a seismic method that comprises the steps of generating seismic waves by exploding an explosive composition in a subterranean formation, and detecting the seismic waves and/or reflections thereof with seismic detectors. The explosive composition used in this method comprises a first explosive material and an oxidizable metal material. The explosive composition can suitably be placed in a borehole within the subterranean formation, and covered with backfill before being exploded. The explosive composition preferably is essentially nitrogen-free (e.g., the nitrogen content of the explosive composition is less than about 1 wt %, preferably less than about 0.1 wt %).
In one embodiment of the invention, the first explosive material is made by combining solid and liquid materials, for example in the proportions of 20 to 80 wt % solids and 20 to 80 wt % liquids. In this embodiment, it is preferred to combine the oxidizable metal material with the solids of the first explosive material.
The present invention is very well suited for use with first explosive materials that comprise a binary explosive (i.e., two components that are usually non-explosive until mixed together). For example, such a binary explosive can comprise an organic fuel component and an oxidizer component. Therefore, the organic fuel component and the oxidizer component can be transported separately and mixed on-site, reducing the risk of premature explosion. This embodiment can be made even safer by adjusting the arming time of the binary explosive. This can be done by adjusting the weight ratio of the oxidizer component to the organic fuel component. Therefore, the arming time can be made longer, causing the combined material to be non-explosive for a period of time after mixing. This period allows the combined material to be placed in a borehole can covered with backfill prior to the composition reaching an exploitable state.
Examples of suitable organic fuel components include diethylene glycol, ethylene glycol, propylene glycol, and glycerol. Other suitable organic fuel components include trinitrotoluene, dinitrotoluene, nitramines, pentaerythritol tetranitrate, nitrostarch, nitrocellulose, smokeless powders, glycol ethers, glycol ether acetates, formamides, alkanes, polyalcohols and low molecular weight mono-hydroxy alcohols. Nitramines as used herein is a group of organic fuel components comprising, for example, cyclotrimethylenetrinitramine (RDX, also known as hexahydro- 1,3,5-trinitro- 1,3,5-triazine) and cyclotetramethylenetetranitramine (HMX, also known as 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane). However, as mentioned above, nitrogen-free compositions are preferred.
Examples of suitable oxidizer components include ammonium nitrates, alkali metal nitrates, alkaline earth metal nitrates, ammonium perchlorates, alkali metal perchlorates, alkaline earth metal perchlorates, ammonium chlorates, alkali metal chlorates, alkaline earth metal chlorates, and hydrates thereof. Particularly preferred oxidizer components include sodium perchlorate, ammonium perchlorate, potassium perchlorate, potassium chlorate, ammonium nitrate, potassium nitrate and lithium perchlorate hydrate.
In one particular embodiment of the invention, the organic fuel component is a liquid and the oxidizer component is a solid. It is preferred that the organic fuel component has a composition such that it does not freeze above a temperature of about xe2x88x9245xc2x0 C. For example, the liquid organic fuel component can comprise one or more of ethylene glycol, diethylene glycol, propylene glycol, glycerol, formamide, methanol and monoethyl ether.
The explosive composition preferably comprises about 0.5 to 50 wt % oxidizable metal material, more preferably about 10 to 30 wt % oxidizable metal material. Particularly preferred oxidizable metal materials include aluminum, magnesium, boron, calcium, iron, zinc, zirconium, silicon, ferrosilicon, ferrophosphorous, lithium hydride, lithium aluminum hydride, and mixtures or alloys of such metal compounds. Metal particulates are one example of suitable oxidizable metal materials. Optionally, the metal particulate is coated with a coating agent, such as at least one fatty acid or a salt thereof. The presence of the fatty acid can prevent premature oxidation of the oxidizable metal material. In this regard, it is useful to coat the oxidizable metal material with the fatty acid. Stearic acid is particularly preferred for this purpose. In addition, it is possible to treat the metal particulate with a dichromate. Preferably, the metal particulate has an average particle size of less than about 100 xcexcm. More preferably, the metal particulate has an average particle size of less than about 50 xcexcm, most preferably from about 10 xcexcm to about 20 xcexcm. Preferably the metal particulate is an aluminum particulate.
In one embodiment of the invention, the explosive composition is self-disarming (i.e., it becomes non-explosive after a period of time passes). One way this can be accomplished is by using a first explosive material that is water-soluble. This water-soluble first explosive material is placed in a container that is initially watertight but subsequently permits entry of water. As a result, the composition is initially explosive, but if for some reason it is not exploded within a desired period of time (e.g., within three months), water begins to enter the container and dissolve the first explosive material, eventually rendering the composition non-explosive (i.e., disarmed).
One specific embodiment of the invention is a seismic method comprising the steps of generating seismic waves by exploding an explosive composition in a subterranean formation wherein the explosive composition comprises an alkali metal perchlorate, a glycol, and particulate aluminum; and detecting the seismic waves and/or reflections thereof with seismic detectors.
Another aspect of the invention is a geophysical survey system, comprising a seismic energy source that comprises a first explosive material and an oxidizable metal material, as described above, with the seismic energy source being located in a subterranean formation. The system also includes a plurality of seismic detectors that are adapted to detect seismic waves generated when the seismic energy source explodes, and reflections of these waves. The seismic detectors transduce an electrical signal representative of the seismic waves and the reflections of seismic waves they detect. The system can also comprise a data acquisition and processing system that is in communication with the seismic detectors, for example through electrical data cables or by wireless data transmission. The data acquisition and processing system can sample the electrical signals generated by the seismic detectors and produce data representative thereof, for example by sampling and summing the data collected.
The current invention is an improvement on prior seismic methods and geophysical survey systems comprising an explosive seismic energy source, because it comprises an explosive composition that produces more seismic energy and less gas volume. In particular, it is expected that at least some embodiments of the current invention will yield at least 35% more seismic energy per unit mass of explosive composition than is generated by current explosive seismic energy sources, while reducing the gas volume produced by as much as about 45%. Using an explosive composition comprising an oxidizable metal material also facilitates generation of low frequency seismic waves as needed. Furthermore, the borehole in which the explosive composition is placed will not need to be drilled as deep, or less explosive will be required than in conventional methods, thereby providing cost savings.
Another embodiment of the present invention is directed to a method of preparing an explosive composition. Such a method comprises the steps of preparing a solid component that comprises an oxidizable metal material and a solid oxidizer component, preparing a liquid component that comprises at least one liquid from the group consisting of liquid organic fuel components and liquid oxidizer components, and combining the solid component and the liquid component to produce the explosive composition.
The oxidizable metal material of the explosive composition is as described above, and the solid oxidizer component comprises at least one of the oxidizer components described above. The solid component can further comprise at least one solid selected from the group consisting of solid organic fuel components and solid additives. Examples of solid organic fuel components that could be used are trinitrotoluene, dinitrotoluene, nitramines, pentaerythritol tetranitrate, nitrostarch, nitrocellulose, and smokeless powders.
The liquid organic fuel component comprises at least one liquid from the group of organic fuel components described above, but can also further comprise water. The liquid oxidizer component comprises at least one aqueous or non-aqueous solution of the oxidizer components described above.
The combining step can be performed at or in close proximity to the site at which the explosive composition is to be exploded. The liquid component and solid component can be combined within a shaped-charge container, or alternatively, the liquid component and solid component can be combined in a first container and then transferred to a shaped-charge container. Both the liquid component and the solid component can be non-explosive materials before they are combined, and the solid and liquid components can be transported to the vicinity of the site at which the explosive composition is to be exploded before the components have been combined. The explosive composition can be self-disarming and its arming time can be adjusted.
An additional benefit of the current invention is that explosive compositions used in the seismic methods and the geophysical survey systems can be shaped to give directivity to the propagated seismic energy. In addition, in at least some of the preferred embodiments of the current invention, components of the explosive composition are shipped separately as non-explosive substances to the geophysical survey site, where they are mixed to yield an explosive composition. This reduces the risk of injury due to premature explosion. In one specific embodiment of the invention, even after mixing the components on site, the explosive does not become armed until the solid component thereof becomes hydrated as a result of gradual diffusion of water into the solids after the solid and liquid components are mixed. This makes the composition even safer to use, by ensuring that the explosive will not become armed until after it is buried in the ground.